BALTIMORE (WJZ) — Hundreds of dead dolphins have washed ashore on the Chesapeake Bay and down the East Coast. Now scientists know what may be causing it: a dangerous virus that could spread even further.

Kai Jackson has more from the federal investigation.

Learning what’s been killing the dolphins is only half the picture. Scientists still have to determine what’s making them vulnerable to this disease in the first place.

Hundreds of dolphins were found dead along the East Coast–including Maryland–this summer, due to a mysterious illness that’s been plaguing Atlantic bottlenose dolphins.

“Likely cause of the mass strandings along the mid-Atlantic is morbillivirus,” said Maggie Mooney-Seus, NOAA.

The National Oceanic and Atmospheric Administration, which has been coordinating the research, released the cause of the dolphin deaths on Tuesday. It’s called morbillivirus. Simply put, it’s the equivalent of human measles.

Mooney-Seus spoke with WJZ Tuesday via Skype.

“These animals, dolphins, carry the virus in their system and when their immune system gets compromised, they become more vulnerable to the spread of the disease,” Mooney-Seus said.

Since July 1, 333 dolphins have died from New York to Virginia. While scientists have solved the mystery of what’s killing them, the next puzzle to piece together is what’s making the mammals vulnerable to morbillivirus in the first place.

“It’s a first wave of testing that we’ve completed and now we have to do some genetic testing that can take weeks, if not months, to get at what’s really behind this,” Mooney-Seus said.

Marine experts say if you encounter a stranded dolphin, alive or dead, don’t approach it. You should contact wildlife authorities.